This invention is related to the field of electrical circuits for generating output signals at a selected frequency, or repetition rate, relative to an input signal. More particularly, the invention is related to the field of electrical circuits for converting an input signal having a first frequency to an output signal having a second frequency. More particularly still, the invention is related to the field of electrical circuits for converting a first signal occurring at a relatively low frequency to a second signal occurring at a relatively high frequency which is a predetermined and selected multiple of the low frequency. Specifically, the invention is directed to frequency locked pulse rate multiplying circuitry for generating an output pulse train having a frequency which is a selected multiple of the frequency of an input pulse train, wherein the number of output pulses is exactly the selected multiple of the number of input pulses and the pulses in the output pulse train are preferably uniformly distributed in time.
A specific application of the pulse rate multiplying circuitry of the invention is in connection with increasing the resolution of a measurement signal from a fluid metering device, or flow meter. The measurement signal from a flow meter is typically a series of pulses, and each pulse represents a discrete volume of liquid or gas. The technique of multiplying the frequency of pulses generated by a pulse generator driven by a flow meter for increasing the resolution of the measurement signal is disclosed, for example, by Gass et al U.S. Pat. Nos. 3,743,946, Grob U.S. Pat. 3,745,470, and Mueller U.S. Pat. 3,808,543.
Although the use of pulse rate multiplying circuits is known for increasing the resolution of the measurement signal generated by a flow meter, known pulse rate multiplying circuits do not always operate with perfect precision. That is, known pulse rate multiplying circuits do not always generate exactly the selected multiple of the number of pulses in the input pulse train generated by the flow meter. Since the selected multiple of pulses does not appear in the output pulse train for each pulse in the input pulse train, the resulting measurement signal is not accurate. Furthermore, known pulse rate multiplying circuits generally do not generate an output pulse train having pulses uniformly distributed in time. That is, the output pulse train is a series of high frequency bursts of pulses separated in time. Consequently, the equipment connected to the pulse rate multiplying circuit must be relatively sophisticated since an extremely fast response time is required for handling bursts of output pulses at high frequency.
In the past, frequency multiplication has primarily been concerned with handling sinusoidal signals wherein harmonics of the original sinusoidal signal are generated and tuned circuits are used for selecting particular harmonics. However, when dealing with pulses, such an approach has proven unsatisfactory because a modification of the sinusoidal signals to squarewave is required. Also, variations in the original signal frequency as encountered in fluid metering render such an approach ineffective.
With the advent of digital circuitry, however, there has been some refinement of pulse rate multiplying techniques. Bauer U.S. Pat. No. 3,617,902, for example, discloses a pulse frequency multiplying circuit including flip-flops 1 and 2 which form a phase comparator for detecting the time of occurrence of each input pulse relative to the time of occurrence of each pulse generated by a divide-by-N counter 26 which counts pulses generated by a voltage controlled oscillator 10. The phase comparator determines the voltage applied by a differential integrating circuit 21 to the voltage controlled oscillator so that the voltage controlled oscillator generates pulses having a frequency which is a selected multiple of the frequency of input pulses, the multiple being determined by the count N preset into the divide-by-N counter. The Bauer circuit generates an output pulse train having a frequency which is a selected multiple of the frequency of the input pulse train and will function effectively under steady state conditions but does not always generate exactly a selected multiple of output pulses for each input pulse since there is no means for determining when the number of pulses generated by the divide-by-N counter equals the number of input pulses. Therefore, the Bauer circuit cannot be satisfactorily used where extreme accuracy is required as in the case of increasing the resolution of a measurement signal from a flow meter.
Lougheed U.S. Pat. No. 3,673,391 discloses a pulse frequency multiplying circuit including an up/down binary counter 13 incremented by input pulses and decremented by pulses generated as follows. A comparator 12 detects when the count contained in a binary counter 11 reaches the count contained in the up/down binary counter for generating a pulse which triggers a monostable circuit 15. The monostable circuit is connected to a divider 18 which is in turn connected to the up/down binary counter so that the pulses generated by the divider occur at a frequency which is dependent on the count preset into the divider. Consequently, the monostable circuit generates output pulses at a frequency which is a selected multiple of the frequency of input pulses, the multiple being determined by the count preset into the divider. Although the number of pulses in the output pulse train equals a selected multiple of the number of pulses in the input pulse train, the pulses in the output pulse train are not uniformly distributed in time. The equipment connected to the Lougheed circuit must be relatively sophisticated since an extremely fast response time is required for handling bursts of output pulses at high frequency generated by the Lougheed circuit.
Also, Kizler et at U.S. Pat. No. 3,935,538 discloses a pulse frequency multiplying circuit including digital multiplying circuitry 17 for generating an output pulse train having a frequency which is a selected multiple of the frequency of the input pulse train. The ratio between the frequency of the output pulse train and the frequency of the input pulse train is determined by the count preset into a factor set store 19. Although the number of pulses in the output pulse train is a selected multiple of the number of pulses in the input pulse train, as in the case of the Lougheed circuit, the pulses in the output pulse train are not uniformly distributed in time, and, therefore, the equipment connected to the Kizler circuit must be relatively sophisticated since an extremely fast response time is required for handling bursts of output pulses at high frequency generated by the Kizler circuit.